1. Field of the Invention
The present invention relates to a position detecting system, a responder and an interrogator, a wireless communication system, a position detecting method, a position detecting program, and an information recording medium. More particularly, the present invention relates to a position detecting system that includes a responder (a wireless tag) and an interrogator that exchange information with each other through wireless communication to detect the position of each other, a position detecting method to be utilized in the position detecting system, a wireless communication system that includes the position detecting system, a position detecting program for use in the position detecting system, and an information recording medium on which the position detecting program is recorded.
2. Discussion of the Related Art
In recent years, so-called wireless tags that are formed with IC chips of 1 mm square or smaller, including wireless transmission/reception antennas and memories and the like, have been gradually put into practical use. More specifically, wireless tags are attached to commercial products to be distributed in the general market, and are distributed together with the commercial products or the like. Identification information for identifying each commercial product is stored in the memory of each corresponding wireless tag. When an interrogating signal is received from an independent interrogator via the antenna, a response signal containing the identifying information is generated in response to the interrogating signal, and is transmitted to the interrogator via the antenna. In this manner, the commercial products or the likes can be identified. Using such wireless tags, consumers can check the distribution channels of food articles from the place of production at the time of purchasing the food articles, and can also check an usage of additives and agricultural chemicals were used during the production of the food articles.
Meanwhile, the UWB (Ultra Wide Band) method for performing wireless communication using pulse waves of very short durations (1 nanosecond or less), instead of carrier waves, has also been gradually put into practical use. By this wireless communication method, pulse waves of very short durations are used, as described above, so that ultra-wideband wireless communication using band widths of several GHz or more can be performed. Also, since carrier waves are not used, the transmission output is approximately 10 nanowatt/MHz, which is very low. The UWB method which has such a configuration has the following characteristics and advantages, and is expected to be utilized in in-room communication, security sensor, high-speed wireless LAN (Local Area Network), or the like.
1) Since the power spectral density is very low in such a system according to the UWB method, there is little mutual interference with existing communication systems, and the system has high capability of coexisting with them.
2) The mean power level is 1 milliwatt or lower, and transmission can be performed in several miles or longer;
3) As pulse waves of very short durations (nanoseconds) are used, a system according to the UWB method can cope with so-called multipath propagation, having high path dividing capability, and it can perform high-precision distance measurement (in several centimeters) as a radar.
4) Since carrier waves are not necessary and the emission time for pulse waves is very short, a small-sized communication system with lower power consumption can be produced.
5) Since it always occupies a GHz wide band, high-speed (<several hundreds of Mbits/second) transmission of large-capacity and multiple access type can be performed.
6) As communication and distance measurement can be performed at the same time, it can be applied to vehicle-to-vehicle communication, for example.
Examples of general configurations to which the UWB wireless communication is applied are disclosed in Japanese Patent Application National Publication No. 10-508725; Japanese Unexamined Patent Publication Nos. JP-A-2003-189353, JP-A-2003-124844; and JP-A-2002-43849, which are hereby incorporated by reference.
A technique of detecting the distance to a moving body through wireless communication is disclosed in Japanese Patent No. 3395403, FIGS. 1 and 2 which is hereby incorporated by reference.
In a identification system containing the above-described conventional wireless tags, however, the distance from a wireless tag to an interrogator cannot be measured with adequate precision, though commercial products with the wireless tags can be distinguished from the other commercial products. The position of a wireless tag in the same room cannot be accurately detected either.
Meanwhile, with a radar using the UWB wireless communication, the distance to an object to be measured can be accurately detected, but the object to be measured cannot be distinguished from other objects.
By the technique disclosed in Japanese Patent No. 3395403, carrier waves are used to detect the distance to a moving body. Therefore, a small-sized system with low power consumption like a wireless communication system of the UWB method cannot be obtained.